1. Field of the Invention
This invention relates to carburetors, and more particularly to an improvement of a carburetor provided with an air vent (or a communication mechanism) having one end opened in the float chamber and the other end opened in the air horn section provided upstream of the venturi to make the pressure in the float chamber substantially equal to that in the air horn section.
2. Description of the Prior Art
In a carburetor of a type in which one end of one of the air vent tubes is provided close to the fuel opening connected to the fuel supply source in the float chamber, through which opening fuel is dropped in accordance with the engine operating conditions, misfire may occur in the engine by the abnormally rich air-fuel mixture when the engine is stopped after a high load operation and then started within a short period of time and operated again under high load.
The inventors have conducted systematic experiments and analyzed this phenomenon as follows.
Fuel bubbles are formed above the liquid surface in the float chamber during the high load operation probably for the reasons below:
(a) During the high load operation, the temperature of the fuel is increased, resulting in that the low boiling point components in the fuel are boiled to form a number of fuel bubbles. PA1 (b) In a carburetor of a type in which fuel is fed from upward (as shown in FIGS. 1(A) and 1(B) to be described below), fuel bubbles are formed when fuel is dropped through the needle valve and collided with or splashed on a plate for pushing up and down the needle valve and for supporting the float. This is particularly true when fuel is dropped in a large amount under high load. PA1 (c) Due to vibration of the engine, the liquid surface in the float chamber is waved to cause the float together with its supporting plate (onto which fuel is dropped) to vibrate. This is also considered to promote the formation of fuel bubbles.
These fuel bubbles each having a bubble wall occupy a large space above the liquid surface as compared with the mere fuel vapor. Further, with the increase in the temperature within the float chamber, such fuel bubbles will be enlarged.
When the engine is stopped after a high load operation, restarted within a short period of time and reaches the high load state again, a large amount of fuel will be fed over the fuel bubbles thus formed. In this case, if an air vent tube is provided close to this place, i.e. close to the fuel opening, the fuel fed in liquid state over the walls of fuel bubbles, as well as fuel bubbles themselves, flow out through the air vent tube. As a result, the air-fuel mixture becomes extremely rich, causing misfire in the engine.